Add comment explaining early guard

esphome/components/dfplayer/dfplayer.cpp

@@ -132,9 +132,8 @@ void DFPlayer::send_cmd_(uint8_t cmd, uint16_t argument) {
 }
 
 void DFPlayer::loop() {
-  // Read message
-  // All current UART available() implementations return >= 0,
-  // use <= 0 to future-proof against any that may return negative on error.
+  // Early return avoids stack adjustment for the batch buffer below.
+  // loop() runs ~7000/min so most calls have nothing to read.
   int avail = this->available();
   if (avail <= 0)
     return;